Bone marrow is the primary site of B lymphocyte formation in mammals. Pre-B cells, which contain intracytoplasmic IgM (cIgM) but lack surface immunoglobulin (sIg) detectable by immunofluorescence, reside in bone marrow and are believed to be precursors of B cells. To test this hypothesis I will develop an in vitro assay of normal human B cell precursors using autoradiography, rosette-formation, and immunofluorescence techniques. The capability of bone marrow cells lacking sIg to express these receptors and develop responsiveness to B cell mitogens will be analyzed and compared to sIg cells from peripheral sites where pre-B cells are not normally found. The existence of human B cell precursors other than pre-B cells will be sought by subjecting cultured cells from different anatomic sites to an analysis of cytoplasmic and surface membrane phenotype, cell cycle kinetics, and sensitivity to the microenvironment provided by accessory cells. These studies are aimed at developing new tools for analyzing the cellular defects which occur in those immunodeficient patients who manifest defective production of B lymphocytes in vivo.